[LAD] Is -ffast-math safe for audio?

[Robin Gareus]

On 11/23/2018 01:00 PM, Will Godfrey wrote:
[...]
> Thanks for going into this in such detail Robin. I never realised fp stuff
> could be *quite* so, umm, approximate!

Depending on context and the maths, the difference may not matter at
all, or may be off completely..

  float a = (1 + 1e20) - 1e20;
  float b = 1 + 1e20; b -= 1e20;
  printf ("%f %f\n", a, b);

Any guesses what this code prints without compiling it? :)

> I was using O3. Following on from this I found that if I removed either O3 or
> ffast-math the problem disappeared.

OK, so SSE/AVX/compiler-intrinsics are the issue at hand.

> It's quite possible that I'm over-thinking this as there are actually only a
> few iterations before the initial figures are re-calculated, but I don't like
> mysteries :(
> 

You could also disable this for a specific function only

__attribute__((optimize("-fno-fast-math")))
void different_mysteries_here (float val)
{
    ...
}

ciao,
robin